To reproduce...

1. Create a course with a Turnitin assignment in it
2. Back up the course
3. Restore the course

An error saying that the same turnitin assignment cannot be linked twice is produced and the course is corrupted.

The error makes a sort of sense but the behaviour is not acceptable. If assigments cannot be restored then Turnitin should not participate in the backup and restore in the first place. Better yet, it should create a properly linked "new" instance.